PROJECT SUMMARY Chronic pain is a devastating public health issue that affects >20% of all adults in the United States and costs ~$600 billion annually, more than any other medical condition. Nearly 27 million US adults have osteoarthritis, a chronic and progressive disease for which pain is the primary symptom, making it a common cause of chronic pain, and a leading cause of disability in the US. The management of pain has often relied on opioid- based, pharmacologic interventions, which not only lack long-term efficacy but also carry risks for adverse events and contribute to the national epidemic of opioid misuse. The identification and development of novel pain management strategies for the treatment of chronic pain, therefore, is a high priority and an unmet need. Although both THC (the major constituent of cannabis) and CBD exhibit antinociceptive effects?particularly for chronic inflammatory pain, arthritic pain, and neuropathic pain?CBD holds particular promise as it lacks psychoactive and addictive properties. Preclinical studies showed that CBD has a wide range of reported pharmacological effects such as anticonvulsant, analgesic, anxiolytic, anti-in?ammatory, hypnotic, antipsychotic and neuroprotective actions; however, the exact mechanisms of action for these effects have not been examined in chronic OA pain. The proposed study will be the first PET imaging study to determine the key targets of CBD that are related to its mechanisms of action on pain treatment in OA. The goal of this study is to bolster the evidence base and understand the underlying mechanisms of action of CBD by identifying the neurochemical and behavioral alterations induced by chronic pain in OA and their modulation by CBD, alone or in combination with other drugs. In this proposal, we will first identify the neurochemical alterations induced by chronic pain in an OA mouse model, as compared to controls, via state-of-the-art neuroimaging studies. We will then evaluate the CBD modulation on the neurochemical and behavioral alterations in OA animals, via neuroimaging studies and behavioral assessment, with an extensive pharmacological, mechanistic-driven approach to identify/confirm the mechanisms of action of CBD in OA. By doing so, CBD modulation for neurochemical changes (mechanisms of action) can be directly linked with its analgesic properties, as reflected in behavioral changes. This strategy will ultimately provide a strong evidence base to identify/confirm the underlying mechanisms of action of CBD as a potential therapeutic for chronic pain in OA. These comprehensive sets of neuroimaging and behavioral studies designed to identify the most valid mechanistic marker attributed to the therapeutic effects of CBD (or a set of mechanistic markers that form a ?signature? for OA) will guide us to fine-tune further mechanistic studies for future clinical trials in patients with OA.